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Plant Physiology 73:648-651 (1983)
© 1983 American Society of Plant Biologists

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Articles

Analysis of Acetylene Reduction Rates of Soybean Nodules at Low Acetylene Concentrations

Robert F. Denison1, P. Randall Weisz and Thomas R. Sinclair

Environmental Physiology Project, United States Department of Agriculture—Agricultural Research Service, University of Florida, Gainesville, Florida 32611, Environmental Physiology Project, Agronomy Physiology Laboratory, University of Florida, Gainesville, Florida 32611

It has been previously proposed that acetylene reduction data at subsaturating acetylene concentrations could be interpreted by use of the Michaelis-Menten equation, based on the acetylene concentration external to the nodules. One difficulty of this view is that the assumption that the system is not diffusion limited is violated when studying intact nodules. The presence of a gas diffusion barrier in the nodule cortex leads to an alternate expression for the gas exchange rates at subsaturating gas concentrations. A theoretical comparison of the `apparent' Michaelis-Menten model and diffusion model illustrated the difficulties observed in the former model of overestimating the Michaelis-Menten coefficient and yielding a correlation between the Michaelis-Menten coefficient and the maximum rate. On the other hand, use of a diffusion model resulted in (a) estimates of the Michaelis-Menten coefficient consistent with enzyme studies, (b) stability of the estimates of the Michaelis-Menten coefficient independent of treatment, and (c) a sensitivity of the diffusion barrier conductance to plant drought stress. It was concluded that all studies of nodule gas exchange need to consider possible effects caused by the presence of a diffusion barrier.

1 Present address: Department of Agronomy and Range Science, University of California, Davis, CA 95616.






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Copyright © 1983 by the American Society of Plant Biologists